Method of and apparatus for developing electrostatic images



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L. E. SHELFFO ET L METHOD OF AND APPARATUS FOR DEVELOPING ELECTROSTATICIMAGES Sept. 8, 1970 Filed Oct. 20, 1966 Sept. 8, 1970 5 SHELFFQ ET AL3,527,651

METHOD OF AND APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES Filed Oct.20, 1966 2 Sheets-Sheet 1 PRINT DEN-5] 7') 1.06 P5515 m NCE or MIXBSQUK. 24W

United States Patent 3,527,651 METHOD OF AND APPARATUS FOR DEVELOPINGELECTROSTATIC IMAGES Loren E. Shellfo, Palatine, and Norman B.Sackrison,

Des Plaines, Ill., assignors to Addressograph-Multigraph Corporation,Mount Prospect, 11]., a corporation of Delaware Filed Oct. 20, 1966,Ser. No. 588,209 Int. Cl. G03g 13/08; G01r 27/02 US. Cl. 11717.5 11Claims ABSTRACT OF THE DISCLOSURE An arrangement for maintaining apredetermined ratio of carrier particles to toner powder in a tonermixture used in the developing assembly of an electrostatic copyingmachine. The arrangement comprises means for sensing the electricalresistance of the mixture and a control circuit connected to the sensingmeans for monitoring changes in the resistance of the mixture and, inresponse to such resistance changes, to provide the replenishment oftoner powder.

This invention relates to a photoelectrostatic copying machine and, moreparticularly, to a new and improved developer unit for such a machineand a method of developing copies.

In the photoelectrostatic art the reproduction of a graphic subject ontoa photoelectrostatic member involves the steps of imparting asensitizing electrostatic charge to the member and exposing the chargedmember to a pattern of light and shadow corresponding to the originalsubject. The exposure results in a latent electrostatic image which isthen developed by the application of an electroscopic powder or tonerwhich adheres in the image areas and is fixed thereon by heat, pressureor solvent vapor technique.

The techniques for applying the toner in photoelectrostatic equipment inuse today require the use of a carrier medium such as glass beads, ironparticles, silaceous materials which triboelectrically hold theflocculent toner powder so that it may be brought into direct andintimate contact With the image. Contact by the carrier and tonermixture with the image may be accomplished by cascading over the image,or may be applied by the conventional magnetic brush technique usingmagnetically attractable carrier particles such as iron, magnetites orthe like.

The production of quality prints of optimum density depends, among otherthings, on the proper ratio of toner to carrier. Since the toner isselectively attracted to the image the copy making process will steadilydeplete toner from the mix. Obviously, it must be replenished and inamounts that restore the proper ratio. An overtoned or undertoned mix isa serious problem which frustrates the objective of turning outsuccessive copies with uniform print or copy density.

Heretofore, replenishment was accomplished by the operator manuallyadding toner from a dispenser at regular intervals. This is not anexpedient solution for many reasons including the time required, thenature of the materials to be handled, the frequency at which the tonermust be replaced to maintain uniform print density, and the fact thatthe actual usage of toner by the copying machine is a function of thetype of copy and the number of copies and not related to a timeinterval. In an attempt to avoid these deficiencies, a number of copyingmachines include arrangements for counting the number of copies producedor measuring the length of copy material produced and using theseconditions as a means for controlling the automatic metering ordispensing of toner material into the developer mixture. Although theseautomatic arrangements represent a substantial improvement, they arestill subject to a number of disadvantages.

As an example, the units that count the number of copies produced arenot capable of taking into account the varying image areas developedfrom one copy to the next. Unless each original to be reproduced hasapproximately the same amount of toner consumption, re plenishment basedon the number of copy sheets going through will not be eifective inmaintaining the proper ratio of carrier to toner. Similarly, dispensingdevices dependent on the length of copy material going through suifersfrom the same deficiency.

Attempts have been made to correlate the opacity of the developer mixdeposited on a conductive image pattern, which is connected to a powersource to electrically bias the pattern so that it attracts an amount ofelectroscopic powder, in direct relation to its concentration in the mixand the amount of bias. Such a technique is quite complex and requiresstrict control over other variables that may affect the final reading,such as cleaning of the optical system, particle size control and'thelike.

A genuinely automatic control that can sense and correct for variationsin toner concentration based on an electrical condition of the mix thatis independent of the number of copies, length of copy sheet or withouthaving to simulate image reproduction as the determining step, hasheretofore been unavailable. The method and apparatus of this inventionmakes possible such a monitoring system by sensing the averageresistance of the developer mix which has been found to vary directly asa function of the ratio of the carrier to toner present in the mix. Ithas been found that as the amount of toner decreases the resistance ofthe mix decreases and the relationship will be true for most systems ofcarrier and toner recognizing that the slope of the relationship willchange for different systems.

Accordingly, one object of the present invention is to provide a new andimproved photoelectrostatic copying machine.

Another object is to provide a photoelectrostatic copying machineincluding new and improved developer means.

Another object is to provide a new and improved method of developingcopies to obtain uniform print density.

A further object is to provide a new and improved developer unitincluding means for automatically maintaining a predetermined ratio ofcarrier material to toner material.

A further object is to provide a new and improved developer unitincluding means for sensing the actual ratio of carrier material totoner material and for automatically dispensing toner material inaccordance with a produced signal until the ratio sensing control issatisfied.

A further object is to provide a new and improved developer unit of thetype having a developer mixture of electrically conductive andinsulating particles in which means are provided for sensing the ratioof the two types of particles in the mixture and for automaticallyadding a quantity of the insulating particulate material to maintain thedesired ratio of materials in the mixture.

A further object is to provide a magnetic brush developer using adeveloper mix of powdered iron and toner powder in which the electricalresistance of the mixture is measured and used to control the additionof toner powder to the mixture.

Another object is to provide a magnetic brush developer assembly inwhich toner material is selectively added to the developer mix bymeasuring the electrical resistance of the portion of the mix that hasbeen formed into a bristle-like mass.

A further object is to provide an assembly for accurately measuring theelectrical resistivity of a mass of powdered iron and toner powder usedto develop electrostatic images.

In accordance with these and many other objects, an embodiment of theinvention comprises a photoelectrostatic copying machine of the type inwhich a latent electrostatic image developed on a surface is developedand placed in visible form by a developer unit using a mixture ofcarrier particles and powdered toner. The developer unit places themixture of carrier particles and toner powder in contact with or inproximity to the electrostatic image, and the toner material isselectively deposited on the surface in accordance with theelectrostatic image to provide a visible image that is subsequentlyplaced in permanent form.

In one form, the developer unit can comprise a magnetic brush developerassembly comprising a rotating cylinder supplied with a developermixture which is comprised of electrically conductive, powdered ironcarrier particles and toner powder formed of a substantially insulatingresinous material. During a portion of its path of rotation, thecylinder or drum is subjected to the infiuence of a magnetic field whichforms the mixture into a bristle-like mass covering a portion of theouter surface of the cylinder. The imaged surface is moved in adirection substantially transverse to the axis of rotation of thecylinder so that the bristle-like mass is moved into contact with theimage surface to transfer toner material to the surface in accordancewith the electrostatic image thereon. This transfer of the toner powderfrom the de veloper mixture to the image surface depletes the supply oftoner material available for developing and will tend to produce lightor undertoned copies upon continued use of the developer unit.

To provide means for automatically maintaining a desired ratio betweencarrier material and toner material, the improved developer unit of thepresent invention includes means for measuring the electricalresistivity of the developer mix correlated to the ratio of carriermaterial to toner material. This resistivity measuring means isconnected to and controls the toner material dispensing assembly so thatthe dispensing assembly adds toner material to the mixture to maintainthe ratio of toner material to powdered iron which provides the desiredcopy or print density. The sensing circuit is designed to measure theresistivity of that portion of the mix that has been formed into thebristle-like formation on the drum or cylinder of the developer unit.However, the resistivity of the mix can be measured anywhere in thedeveloper unit.

Many other objects and advantages of the present invention will becomeapparent from considering the following detailed description inconjunction with the drawings in which:

FIG. 1 is a graph illustrating a typical relationship between printdensity and resistance of the powdered irontoner mix in a magnetic brushdeveloper unit;

FIG. 2 is a schematic circuit diagram of an arrangement for measuringthe resistivity of the developer mix and for automatically controllingthe addition of toner material to the mix;

FIG. 3 is a fragmentary sectional view of a magnetic brush developerunit with which the present invention can be used; and

FIG. 4 is a fragmentary sectional view illustrating the relationshipbetween the portion of the magnetic brush developer and an electrodeforming a part of the resistivity measuring circuit.

Referring now more specifically to the drawings, therein is illustrateda developer unit (FIG. 3) of the magnetic brush type which is designedand adapted for use in a photoelectrostatic copying machine and which isadapted to place in visible form a latent electrostatic image developedon a surface by the copying machine. The developer unit 10 uses adeveloper mix of carrier particles, such as finely divided iron,magnetite or ferrite, and a particulate toner powder consisting of, forinstance, dyed or pigmented resin powder. The developer mix ismagnetically formed into a bristle-like formation and brushed againstthe imaged surface to develop the latent electrostatic image by theselective attraction of the toner powder to the surface. Arepresentative magnetic developer mix is described in detail in U.S.Pat. No. 3,320,169, assigned to the same assignee as the presentinvention.

To avoid any change in the intensity or density of the printed copyresulting from depletion of the consumable toner powder in the developermix, the present invention includes a detecting or measuring circuitindicated generally as 12 (FIG. 2) for measuring the resistivity of themixture and for automatically controlling the addition of toner powderto the developer mix in accordance with the changes in the resistivityof the developer mixture. Since the magnetic carrier particles areelectrically conductive and since the toner powder has a substantiallyinsulating characteristic, the ratio of carrier material to toner powdercan be accurately calibrated in a particular system to the resistivityof the mix. Thus, it has been determined that it is possible toautomatically control the addition of toner material to the developermixture in dependence on variations in the resistivity of the mixtureand thereby insure uniform density of the prints produced by the copyingmachine.

Referring now more specifically to FIG. 3 of the drawings, therein isillustrated the magnetic brush developer unit 10 which can be of anysuitable construction. The illustrated magnetic brush assembly is of thegeneral type shown and described in detail in U.S. Pats. Nos. 3,003,462and 3,145,122. As illustrated therein, the unit 10 comprises a housingforming a receptacle 14 containing a quantity 16 of magnetic developermix of the composition referred to above. A pair of driven conveyorscrews, only one of which -18 is illustrated, distributes and mixes thedeveloper mixture so that an adequate supply of the developer mix ofuniform consistency is available along the entire length of an elongateddrum or cylinder 20 which is mounted for rotation about an axis spacedabove the lower wall of the trough or receptacle 14. A magnetic fieldproducing means 22, such as one or a plurality of permanent magnets, isdisposed within the drum 20 adjacent a portion of the periphery thereofso that during rotation of the cylinder 20, a portion of the developermix 16 is formed into a bristle-like formation indicated as 24 (FIG. 4)and brushed into contact with the surface containing the electrostaticimage to be developed. Although this image bearing surface can be of anysuitable construction, the illustrated developer unit 10 is adapted foruse with sheet or web copy material having a photoconductive surface onwhich the electrostatic image is directly produced. A pair ofnap-covered rollers 26 (FIG. 3) of the type shown and described indetail in U.S. Pat. No. 2,894,744 provide part of a means for feedingthe image bearing sheet or web past the magnetic brush.

Since the development of imaged surface by the developer unit 10 resultsin the removal of toner powder from the unit 10 and from the developermixture 16 therein, the density or contrast of the copies produced bythe copying unit 10 is reduced as the ratio of carrier particles totoner powder in the developer mixture 16 increases with usage of themachine 10. To avoid this reduction in print density, the unit 10includes a toner dispensing assembly indicated generally as 28 forautomatically adding toner material to the mixture 16. The constructionand operation of the dispensing assembly 28 is shown and described indetail in U.S. Pat. No. 3,224,- 649. In general, the dispensing assembly28 includes a hopper or receptacle 30 which contains a quantity of tonerpowder and which has a lower opening in which is disposed a plurality ofspheres 32 resting on a somewhat inclined and generally V-shaped trough34. An electric solenoid 36 is coupled to the trough 34 so that uponenergization of the solenoid 36, the trough 34 is oscillated or vibratedto dispense toner material from the receptacle 30 to the receptacle 14containing the supply of developer mix for the developer unit 10.

Although the dispensing assembly 28 can be controlled by a number ofconditions which indicate the rate at which the toner material in thedeveloper mix is being depleted, such as the number of copies run or therunning length of copy material developed, these factors are, at best,only approximations of toner usage, and optimum uniformity of printdensity would be obtained by controlling the dispensing assembly 28 indirect response to the actual ratio of carrier material to toner powderexisting in the developer mixture 16. In accordance With the presentinvention, it has been determined that the resistivity of the developermixture 16 varies as a function of the ratio of the carrier material totoner material and that the operation of the dispensing assembly 28 canbe controlled in accordance with the measured resistivity of thedeveloper mixture 16 to effect a stabilization of the carrier-tonerratio and consequent uniform print density of the copies. As an example,with a magnetic developer mix of the type described in detail in theaboveidentified copending East et al. application, it has beendetermined that under a static condition the resistance of the mixvaries from l ohms to 4 1O ohms corresponding to the percentage of tonermaterial in the finely divided iron carrier ranging from 5.5 to 11.1percent by weight. Using the magnetic developer mix described in thecopending East et al. application, under conditions of making copies, ithas been determined that resistance values of from 2x10 ohms2 10 ohmswill provide properly toned copies for that particular mix.

As an example, FIG. 1 plots the relationship between the log of theresistance of a representative developer mixture 1 6 and print density.Print density represents reflected light intensity measured on aphotovoltometer in which the optimum value for copies is 1.0. From thecharacteristic curve shown in FIG. 1, it can be seen that the log of theresistance of the mixture 16 required to obtain the optimum printdensity is slightly in excess of 5.0 or a true resistance value somewhatin excess of 150K ohms. Different mixes, that is, mixes which employdifferent carriers or resinous materials, will have a differentresistance range that gives a density of 1.0.

In order to use the measured resistivity of the developer mix as a meansfor controlling the automatic dispensing of toner material from theassembly 28 to the mixture 16 in the receptacle or trough 14, it isessential that accurate and reliable resistivity measurements beobtained. One means for obtaining such resistivity measurements is toinsert two spaced electrodes in the body of the developer mix 16 withinthe receptacle 14, preferably in a location in which the mixture isbeing swept between the electrodes. When using such an arrangement caremust be taken to avoid shunting the resistance path between theelectrodes or one electrode and the frame.

In accordance with the present invention, it has been determined thataccurate resistance measurements can be made by measuring theresistivity of the portion of the mixture 16 that is in the bristle-likeformation 24 on the drum 20. Accordingly, the developer unit 10 isprovided with an elongated and electrically conductive electrode 40(FIG. 4) which is mounted on the frame for the developer unit 10 in aninsulated relation thereto and in a position extending generallyparallel to but spaced from the axis of rotation of the cylinder or drum20. The electrode 40 is so spaced from the drum 20 that it engages theends of the bristle-like formation 24 and serves the dual function ofestablishing a uniform pressure contact with the mixture 16 and servingas a doctor blade to maintain the bristle formation 24 at a uniform sizeprior to movement of the bristle-like formation into engagement with theimaged surface. The US. Patent No. 3,387,586, assigned to the sameassignee as the present invention, discloses an arrangement forsupporting the elongated electrode 40 immediately adjacent the paththrough which the bristlelike formulation 24 on the drum 20 moves. Afurther advantage of the use of an elongated electrode 4.0 contactingthe bristle-like formation 24 as a part of the circuit for measuring theresistivity of the mix is in the fact that the resistivity of the mix 16and thus the ratio of the iron to the toner powder is being measured inthe most important area, i.e., immediately preceding the contactbet-ween the mixture and the imaged surface. Thus, any variation in theratio which might occur in the mixture disposed within the receptacle 14is avoided. In addition, the electrode 40 provides an elongated area ofcontact with the mixture 16 extending along the entire length of thebristle-like formation 24. While the elongated electrode has beenemployed to advantage other electrode configurations may be employed.The use of a rake-type electrode as well as needle-like probes disposedat other vantage points in the developer unit have been eminentlysuccessful.

The control circuit 12 (FIG. 2) is coupled to the electrode 40 to sensethe resistance of the mixture 16 in the bristle-like formation 24 and tocontrol the operation of the dispensing assembly 28 in accordancetherewith. The control circuit 12 includes a grid controlled gaseousconduction device of thyratron 42, the anode of which is connectedthrough a variable resistance 44 to one terminal of a winding 36A forthe operating solenoid 36 in the dispensing assembly 28. The otherterminal of the winding 36A is connected through a time delay device 46to one terminal of a secondary winding 48A on a power transformerindicated generally as 48. A primary winding 48B of the transformer 48is connected to a conventional alternating current potential source. Theother side of the secondary winding 48A is returned to the cathode ofthe thyratron 42. A second secondary winding 48C in the transformer 48is connected to the filament of the thyratron 42, and this winding isalso connected to the heating device of the thermal time delay device 46so that after a time delay interval of, for instance, ninety seconds,suflicient to heat the filament of the thyratron 42, the time delaydevice 46 operates to a closed circuit condition to connect the lowerterminal of the winding 36A to the secondary winding 48A.

To provide means for sensing the resistance of the mix in the portion ofthe developer mix (16 that is in the bristlelike formation 24, a biasingnetwork is connected to the control grid of the thyratron 42. Thisbiasing network includes a direct current potential source or battery50, one terminal of which is connected to a reference potential sourceand the other terminal of which is connected through a variableresistance element 52 to both the electrode 40 and the control grid ofthe thyratron 42. The electrically conductive drum 20 is connected tothe reference potential so that a circuit including the battery 50, thevariable resistance 52, and the resistance disposed between theelectrode 40 and the conductive drum or cylinder 20 provide a voltagedividing network, the center point of which is connected to the controlgrid of the thyratron 42. The electrode 40 is also connected to a bypasscondenser 54, which serves to eliminate short term pulses at thethyratron.

As indicated above, it has been determined that the resistance betweenthe electrode 40 and the electrically conductive drum or cylinder 20provided by the intervening portion of the mix 16 in the bristle-likeformation 24 is normally on the order of 2 10 2 lO ohms when the copiesare properly toned. Thus, the variable resistance element 52 is adjustedso that with this resistance provided by the mixture 16, the potentialapplied to the control grid 42 is sufficiently negative to preventconduction through the thyratron 42 during positive-going cycles of theinput alternating current potential following the clbsure of thecontacts in the time delay device 46. The variable resistance 52 can beadjusted to condition the circuit 12 for use with different developermixes.

When the toner and developer mixture 16 becomes depleted, theresistivity of that portion of the mixture in the bristle-like formation24 decreases so that a less negative or more positive potential isapplied to the control grid of the thyratron 42. This more positivepotential permits the thyratron to conduct during positively poledhalf-cycles and periodically energize the winding 36A of the solenoid36. This intermittent energization of the solenoid 36 vibrates the feedtrough 34 (FIG. 3) so that toner material from the receptacle 30 isdischarged to the receptacle 14 in the developer unit 10. The amplitudeof the excursions of the trough 34 and thus the rate at which the tonermaterial is dispensed can be controlled by varying the resistanceelement 44 to control the current passing through the winding 36A. Thisoperation continues until such time as the toner material added to thesupply of the mixture 16 in the receptacle 14 increases to the desiredlevel. At this time, the resistance between the electrode 40 and thecylinder 20 increases to the point at which a more negative potential isapplied to the control grid of the thyratron 42 and conduction throughthis thyratron is terminated until the supply of toner is againdepleted.

It will be appreciated that the principle of operation of the instantinvention relies on the electrical resistivity of the components thatcomprise the developer mix. The details of the invention have beendescribed in terms of a particular developer mix and the data and rangespresented are intended to be exemplary and not limiting. The principlemay be effectively applied to a wide range of resins combined withcarriers following the teaching of the detailed description hereinwithout departing from the spirit and scope of the invention.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

1. In a copying machine of the type in which a re cording member isimparted with an electrostatic charge and is developed by contact with acharge attractable toner powder, including a mixture of carrierparticles and toner powder combined in a predetermined mix ratio ofparticles to powder, said carrier particles being more electricallyconductive than said toner powder, and a developer assembly forreceiving a quantity of the mixture and for applying the mixture to theelectrostatic image which is developed by the selective adherence of thepow der, the improvement comprising:

sensing means in said developer assembly for sensing the electricalresistance of the mix,

a control circuit connected to said sensing means for monitoring changesin resistance of the mix, and

a toner powder dispensing assembly responsive to said control circuitfor dispensing powder to the developer assembly in response to changesin the mix resistance to maintain the ratio of carrier particles andtoner powder at said predetermined level to provide optimum printdensity.

2. A copying machine as set forth in claim 1 wherein the developerassembly includes a magnetic brush applicator capable of forming saidmix into an elongated brush-like configuration extending generallytransverse to the direction of movement of the member and wherein saidcarrier comprises magnetically attractable particles.

3. A copying machine as set forth in claim 2 wherein the sensing meansincludes an elongated electrically conductive member positioned in thesweep path of the brushlike configuration.

4. A copying machine as set forth in claim 1 wherein the control circuitincludes a controlled conduction device with a control electrode, saiddevice being operable to a first and second state of conduction, and abias network connected to the control electrode to maintain saidconduction device in said first state of conduction so long as theresistance of said mix remains above a predetermined level, saidconduction device being operable to change from said first to saidsecond state of conduction in response to the resistance of said mixfalling below said predetermined level.

5. A copying machine as set forth in claim 4 wherein the dispensingassembly includes an operating solenoid with a winding, and in which thecontrolled conduction device is connected to the winding to control theenergization of said solenoid in response to a change of state ofconduction of said conduction device.

6. A copying machine as set forth in claim 5 further including avariable impedance element connected to the winding of said solenoid tocontrol the amplitude of the current flowing through the winding,thereby to control the rate at which said toner powder is dispensed fromsaid dispensing assembly.

7. In a copying machine of the type in which a recording member isimparted with an electrostatic charge and is developed by contact with acharge attractable toner powder, comprising:

a mixture of carrier particles and toner powder combined in apredetermined mix ratio of particles to powder, and

a magnetic brush assembly for forming bristles of the mixture and movingthe bristles into engagement with the member to develop theelectrostatic image, the improvement comprising:

a control circuit coupled to said magnetic brush assembly for monitoringchanges in resistance of the mix, and

toner powder dispensing means responsive to said control circuit fordispensing powder to the magnetic brush assembly in response to changesin the mix resistance to maintain the ratio of carrier particles andtoner powder at said predetermined level to provide optimum printdensity.

8. In a magnetic developer assembly for developing a latentelectrostatic image on sheet or web material with a mixture of magneticparticles and toner powder comprising:

a receptacle for containing a supply of the mixture,

a magnetic brush assembly including an elongated cylinder rotatableabout an axis and mounted adjacent the receptacle, said brush assemblyincluding magnetic means which form a bristle-like and elongatedcovering on at least a portion of the periphery of the cylinder forapplying mix to the member to be developed during rotation of thecylinder, and

a dispensing assembly for adding toner powder to the receptacle, theimprovement comprising:

an elongated and electrically conductive member mounted adjacent thecylinder and extending generally parallel to the axis of the cylinder,said electrically conductive member being spaced from the outerperiphery of the cylinder in the portion thereof covered by thebristle-like covering and engaging the bristle-like covering, and

a detecting circuit connected to the elongated member and the dispensingassembly for measuring the electrical resistance of the mixture and forselectively operating the dispensing assembly to dispense toner power tothe receptacle in accordance with the measured resistance of themixture.

9. In a copying machine of the type in which a photoconductive member isimparted with an electrostatic charge and is developed by applying amixture of carrier material and toner material combined in apredetermined mix ratio of carrier material to toner material,comprismg:

a magnetic brush assembly for forming bristles of the mixture and movingthe bristles into engagement with the material to develop theelectrostatic image, the improvement comprising:

detecting means for sensing the electrical resistance of the portion ofthe mixture in the bristles, and

dispensing means connected to and controlled by the detecting means forselectively adding the toner material to the mixture in dependence onthe resistance of the portion of the mixture in the bristles.

10. A method of automatically controlling the print density of copiesdeveloped by a mixture of developer material comprising a predeterminedratio of insulating toner powder and electrically conductive carrierparticles, said mixture having a corresponding predetermined resistance,said method comprising the steps of:

monitoring the electrical resistance of the mixture and,

automatically adding toner powder to said mixture in accordance with adecrease in the resistance of the mixture until the resistance reaches avalue corresponding to said predetermined resistance value, therebymaintaining the ratio of insulating toner powder and electricallyconductive particles.

11. A method of automatically controlling the print density of copiesdeveloped by a mixture comprising a predetermined ratio of insulatingtoner powder and elec trically conductive carrier particles, saidmixture a corresponding predetermined resistance, said method com- 20prising the steps of:

forming a portion of the mixture into a brush-like mass, sensing theelectrical resistance of the brush portion of the mixture on the mass,and

automatically adding toner powder to said mixture in 2 accordance with adecrease in the resistance of the mixture until the sensed resistancereaches a value corresponding to said predetermined resistance value,thereby maintaining the ratio of insulating toner powder andelectrically conductive carrier particles.

References Cited UNITED STATES PATENTS 1,717,193 6/1929 Dantsizen 1172262,956,487 10/1960 Giamo 117-17.5 X 3,094,049 6/1963 Shelling 35533,233,781 2/1966 Gruhbs 118-9 X 3,376,853 4/1908 \Veiler et al 11717.5 X3,376,854 4/1968 Kamola 17-l7.5 X 3,387,586 6/1968 Shelifo et al 11717.5X

OTHER REFERENCES Magnetic Ink Hopper Content Control, D. E. Rutter, IBMTechnical Disclosure Bulletin, vol. 3, No. 11, April 1961, pp. 10 and11.

MURRAY KATZ, Primary Examiner P. ATTAGUILE, Assistant Examiner US. Cl.X.R.

